Flight training equipment frequently includes the simulated cockpit of an aircraft. It is customary to mount the simulated cockpit on the platform of a motion base to enable the simulated cockpit to assume attitudes which at least partially simulate the various attitudes and accelerations which the cockpit of the corresponding aircraft can assume in actual flight. A number of motion bases of different capacity and concept are commercially available. A typical example is the motion base shown in U.S. Pat. No. 3,529,354. Since the complexity and hence the size of such simulated cockpits tend to be relatively large, most of the successful motion bases have in common a plurality of powerful hydraulic actuators that must be large enough to supply accelerations to the simulated cockpit sufficient to simulate the actual response of a cockpit in a flying airplane.
The heretofore known motion systems and hydraulic actuators generally have been subject to the problem of noise and jitter of a magnitude that renders them marginally acceptable for man rated simulation motion systems. Even when reduced to a minimum these effects are very distractive to simulator pilots. The basic problem is related generally to limit cycling due to static and coulomb frictions, and hydraulic oil column resonance.
The degree to which such effects can be reduced is limited by the pass band of the servo valve of the actuator, and its ability to provide degeneration of the noise frequencies at the proper phases. The response of servo valves which are large enough to provide the necessary flow to the large hydraulic actuators involved to not possess an adequate pass band to allow proper degeneration of the noise and jitter. Therefore, it has become a common practice among users of such systems to reduce overall servo gain thereby compromising the performance to reduce distracting vibrations within the simulator.